Umami, the detection of amino acids, is one of the five basic tastes. The word umami is Japanese for “delicious taste”; perhaps the best English equivalent is “savory.” It is an appetitive taste, in that it is generally regarded as pleasant. It has likely been advantageous in human evolution by facilitating the detection of foods rich in amino acids, some of which cannot be synthesized by humans and are thus essential components of nutrition. The standard substance used to define umami is L-glutamate. L-glutamate is an amino acid, and as such is abundant in high-protein foods, but it is only sensed in its free form. Therefore, substances that are high in protein that are partially metabolized have the strongest umami taste, including fish sauce, soy sauce, and tomatoes. Monosodium glutamate (MSG) is a salt of L-glutamate and was developed as a food additive to enhance the umami flavor of foods. Another amino acid, L-aspartate, can be sensed as umami as well, but most other amino acids are not detected to an appreciable degree in humans. The narrow specificity of the receptor for glutamate is intriguing, considering that glutamate itself is not an essential amino acid, but suggests that glutamate is sufficiently indicative of the amino acid content of food.
While umami was the most recently recognized primary taste, the umami receptors were identified more readily. The receptor responsible for umami sensation is the heterodimer of Taste Receptor 1 Member 1 (T1R1) and Taste Receptor Member 3 (T1R3; Nelson et al., 2002). Additionally, the metabotropic glutamate receptors mgluR4 and mgluR1 may be involved in umami sensation. The receptors are expressed on the surface of taste receptor cells (TRCs). Only TRCs that express the T1R1+R3 receptor sense umami, and TRCs that detect umami do not detect other tastes. Different TRCs are responsible for all five of the primary tastes and are organized into taste buds, which contain 50—150 TRCs each. Taste buds throughout the tongue contain TRCs for all five primary tastes; therefore, there is no topographic taste map. The specificity of the TRC to one taste alone is essential for the way that the different taste sensations are encoded.
Depolarization of the umami TRCs following a signaling cascade triggered by the receptor transmits a signal to afferent neurons. The signal is relayed by the facial nerve (cranial nerve VII) in the anterior two-thirds of the tongue and the glossopharyngeal nerve (cranial nerve IX) in the posterior third of the tongue. Sensory afferents synapse in the rostral portion of the nucleus of the solitary tract in the brainstem and are relayed to the thalamus with projections to the primary gustatory cortex.
The T1R1+R3 receptor detects glutamate with relatively low affinity when compared to the affinities of the bitter receptors and bitter tastants. This means that substances need relatively high concentrations of glutamate to be sensed as umami, consistent with the need to identify foods with sufficient amino acid content. Though the primary taste of umami is defined as sensing of glutamate, it has been suggested that the full taste classically described as umami is not elicited by glutamate alone, but rather is a complex taste elicited by potentiation from the ribonucleotides inositol monophosphate or guanine monophosphate.
Michael S. Harper
See also: Facial Nerve; Glossopharyngeal Nerve; Supertaster; Taste Aversion; Taste Bud; Taste System; Type II Taste Cells
McLaughlin, Susan K., & Robert F. Margolskee. (1994). The sense of taste. American Scientist, 82(6), 538—545.
Nelson, Greg, Jayaram Chandrashekar, Mark A. Hoon, Luxin Feng, Grace Zhao, Nicholas J. P. Ryba, & Charles S. Zuker. (2002). An amino-acid taste receptor. Nature, 416(6877), 199—202. http://dx.doi.org/10.1038/nature726